Что такое power smoothing
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Что такое power smoothing

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The purpose of the parameter Power smoothing in MyETraining app and Drivo

The purpose of the parameter Power smoothing in MyETraining app and Drivo

Post by michaelwestergaard » Sat Nov 17, 2018 12:02 am

Please can lease you explain the purpose of the parameter Power smoothing in the advanced configuration of the MyETrainings app?

What values are valid to enter for this parameter and how should different values change the way the Drivo hometrainer behaves or feel like?

The reason I ask is that I suspect that my Drivo stops very quickly if no force is applied whereas the "road feel" becomes more natural at speed.

So far I have the feeling that I have to work much harder both when going uphill or downhill when using the Drivo than what i do on the road. I have used the calibration feature on the app., but it does not change anything.

I have also tried to attached a Quarq powermeter to the bike one time and can also put a Stages powermeter on the left arm at the same time if I want to. The power values I get from these devices are not way off either with respect to each other or with respect to the power values I get from the Drivo itself so that seems to be ok. At least not to a degree so I feel there is anything wrong.

If I compare a nice steady climb I did on Teneriffe some years ago for real I have the following real values: Dist.=6,8 km, time: 36:10, avg. speed approx. 11,3 km/h with Pavg =227 W and NP= 231W

If I do the same climb from a MyRealvideo I have obtained the following data from the Drivo / MyETraining app. Dist=6,83 km, time: 36:39, avg. speed approx 11,2 km/h with Pavg =219W and NP=222W.

The climb has slopes around 6-9% most of the time and does not change too much all the way. In fact some Pro Tour teams use the real climb for test purposes.

As the MyRealvideo does not start at exactly the same point where I meassured the climbing part of my real ride there could be a good reason why there is this difference between the real and simulated data. Another fact is that my real data was obtained with an SRM powermeter I had at that time, hence the powerdata from the SRM are not obtained from the rear wheel and would be expected higher than my Drivo data due to loss in the chain / cassette system.

But the feeling I have is that when riding uphill using the Drivo is that It becomes rather hard and unnatural to maintain a steady low pace compared to the real world. If I speed up to a higher pace the feeling becomes more natural.

For instance if I want to climb this course I wrote about with 10-11 km/h I only have to slow down a little bit before it is very hard to get up in speed. And with slopes in the +12% region it quickly becomes unnatrual and nothing that feels like the real world.

It is as if the cadence / speed gets low it almost feels like a standing start on a track bike doing a 4k pursuit or something like that.

The same "quickly brake / stop" feeling occur if I ride a course going downhill. If that is the case it is all most impossible to obtain speeds above 40km/h even if the gradient or slope is — 5 to — 6%. And anybody who have tried to ride down a real -5 % slope knows that you easily get above 50 km/h without doing much work.

Hence I wonder if this behaviour relates to the Power smoothing parameter.

smoothing power

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Что такое power smoothing

In a power supply, whether it is a linear power supply or a switch mode power supply using an AC power source and diode rectifiers, the raw rectified output is normally smoothed using a reservoir capacitor before being applied to any regulators or other similar electronic circuitry.

Aluminium electrolytic capacitors are ideal for for acting as smoothing capacitors as many electrolytics are able to provide a sufficiently high capacitance and withstand the level of ripple current needed to smooth the waveform.

Essentially the smoothing circuit fills in the major dips in the raw rectified waveform so that the linear regulator or switch mode power supply circuitry can operate correctly. They change the waveform from being one that changes from zero to the peak voltage over the course of the cycle of the incoming power waveform and change it to one where the changes are very much less. Essentially they smooth the waveform out, and this gives rise to the name.

As smoothing capacitors are used in both linear regulated power supplies and switch mode power supplies, they form an essential part of many of these electronic circuits.

Full wave rectifier with smoothing capacitor

Capacitor smoothing basics

Capacitor smoothing is used for most types of power supply, whether a linear regulated power supply, a switch mode power supply, or even just a smoothed and non-regulated form of power supply.

Aluminium electrolytic capacitor of the form used in power supply smoothingTypical electrolytic capacitor used for smoothing applications

The raw DC supplied by a diode rectifier on its own would consist of a series of half sine waves with the voltage varying between zero and √2 times the RMS voltage (ignoring any diode and other losses).

A waveform of this nature would not be of any use for powering circuits because any analogue circuits would have the huge level of ripple superimposed on the output, and any digital circuits would not function because the power would be removed every half cycle.

The capacitor smoothing enables the following stages of the linear regulated power supply, or the switch mode power supply to operate correctly.

To smooth the output of the rectifier a reservoir capacitor is used — placed across the output of the reciter and in parallel with the load.

The smoothing works because the capacitor charges up when the voltage from the rectifier rises above that of the capacitor and then as the rectifier voltage falls, the capacitor provides the required current from its stored charge.

In this way the capacitor is able to provide charge when it is not available from the rectifier, and accordingly the voltage varies considerably less than if the capacitor were not present.

The capacitor smoothing will not provide total voltage stability, there will always be some variation in the voltage. In fact the higher the value of the capacitor, the greater the smoothing, and also the less current that is drawn, the better the smoothing.

Smoothing action of a reservoir capacitor

It should be remembered that the only way discharge path for the capacitor, apart from internal leakage is through the load to the rectifier / smoothing system. The diodes prevent backflow through the transformer, etc..

A further point to remember, is that capacitor smoothing does not give any form of regulation and the voltage will vary according to the load and any input variations.

Voltage regulation can be provided by a linear regulator or a switch mode power supply.

Smoothing capacitor value

The choice of the capacitor value needs to fulfil a number of requirements. In the first case the value must be chosen so that its time constant is very much longer than the time interval between the successive peaks of the rectified waveform:

Rload = the overall resistance of the load for the supply
C = value of capacitor in Farads
f = the ripple frequency — this will be twice the line frequency a full wave rectifier is used.

Smoothing capacitor ripple voltage

As there will always be some ripple on the output of a rectifier using a smoothing capacitor circuit, it is necessary to be able to estimate the approximate value. Over-specifying a capacitor too much will add extra cost, size and weight — under-specifying it will lead to poor performance.

Peak to peak ripple for output from smoothing capacitor on a power supply (full wave)

The diagram above shows the ripple for a full wave rectifier with capacitor smoothing. If a half wave rectifier was used, then half the peaks would be missing and the ripple would be approximately twice the voltage.

For cases where the ripple is small compared to the supply voltage — which is almost always the case — it is possible to calculate the ripple from a knowledge of the circuit conditions:

Full wave rectifier

Half wave rectifier

These equations provide more than sufficient accuracy. Although the capacitor discharge for a purely resistive load is exponential, the inaccuracy introduced by the linear approximation is very small for low values of ripple.

It is also worth remembering that the input to a voltage regulator is not a purely resistive load but a constant current load. Finally, the tolerances of electrolytic capacitors used for rectifier smoothing circuits are large — ±20% at the very best, and this will mask any inaccuracies introduced by the assumptions in the equations.

Ripple current

Two of the major specifications of a capacitor are its capacitance and working voltage. However for applications where large levels of current may flow, as in the case of a rectifier smoothing capacitor, a third parameter is of importance — its maximum ripple current.

The ripple current is not just equal to the supply current. There are two scenarios:

    Capacitor discharge current: On the discharge cycle, the maximum current supplied by the capacitor occurs as the output from the rectifier circuit falls to zero. At this point all the current from the circuit is supplied by the capacitor. This is equal to the full current of the circuit.

The shorter charging time gives rise to very large peak current levels as the smoothing capacitor needs to absorb sufficient charge for the discharge period in a very short time.

Pi section smoothing networks

In some applications a linear voltage regulator would not be used, an improved form of smooth could be required. This could be provided by using two capacitors and a series inductor or resistor.

The smoothed power supply approach is used in some high voltage systems and in some other specialist areas, but it is not nearly as common as linear regulated power supplies and switch mode power supplies which provide much better regulation and smoothing.

This approach can also be seen in many vintage wireless sets where the use of a linear regulated power supply was not feasible.

Pi section smoothing filter

There are two options for a π section smoothing system. With two capacitors between the the line and ground, the series element was either and inductor or a resistor. The inductor cost much more and gave better performance, but the resistor was a much cheaper option although it did dissipate more power.

Smoothing capacitors are essential elements of both linear power supplies and switch mode power supplies and as such they are widely used.

When selecting a reservoir capacitor for smoothing applications in a power supply, not only is the value in terms of capacitance important to give the required ripple voltage reduction, but it is also very important to ensure that the capacitor ripple current rating is not exceeded. If too much current is drawn, the capacitor will heat up and its life expectancy reduced, or in extreme cases it could fail, sometimes catastrophically.

All About ERG Mode Power Smoothing On Wahoo KICKR Trainers

One question we see popping up regularly from Zwifters goes something like this:

“I just upgraded from my Wahoo KICKR Snap to (insert non-Wahoo trainer model here). I’ve noticed my power in workouts is must more sporadic now, bouncing up and down but never staying on the target power. Is there a problem with my new trainer?”

Or some sort of reverse version has also been seen:

“I just upgraded my trainer from (insert non-Wahoo trainer model here) to a Wahoo KICKR. I’ve noticed that the KICKR’s power in workouts is nice and steady. Super impressed.”

So what’s actually happening here? Let’s dive in…

Intro To Wahoo’s “Erg Mode Power Smoothing” Feature

Wahoo’s smart trainers include a feature called “Erg Mode Power Smoothing” which is automatically enabled out of the box. It can be toggled off and on easily via the Wahoo app:

Turned on Turned off

What does this feature do? According to Wahoo’s support page, “this setting enables smoothing reported power to eliminate small inconsistencies in pedal strokes, making power graphs appear less erratic. Enabling is a personal preference, will not impact overall trainer functionality, and is not required.”

About Power Smoothing

Power “smoothing” refers to displaying or reporting your average power vs your instantaneous power. You might set your bike computer to display a moving 3-second power average, for example, instead of instantaneous. Bike computers typically offer a wide range of options from 3 seconds up to 20 minutes.

This averaging or “smoothing” results in less of the spiky power numbers which naturally occur due to an uneven pedal stroke, power readings being taken at varying places in the pedal stroke, and generally less-than-rock-steady pedaling behavior.

Smoothing is a feature found in essentially all devices and software which display cycling power. Zwift, for example, lets you choose between “3 sec” and “Instant” power in the main settings menu.

The crucial thing to understand about power smoothing on a display device is that it doesn’t affect your recorded power numbers. It just affects their display as you ride. But that’s not the case with Wahoo’s power smoothing.

Here’s Shane Miller with a quick visual description of what Wahoo’s Erg Mode Power Smoothing does to your workout data:

More On “Erg Mode Power Smoothing”

Wahoo’s Erg Mode Power Smoothing feature is toggled on and off on the trainer itself, so it actually changes the power numbers sent to Zwift (or any other device). That’s right: it’s not just changing the way your numbers appear. It’s actually changing the way your trainer reports power.

There’s a case to be made that enabling this feature is a good thing, which of course is why Wahoo enables it by default. It keeps your on-screen power numbers nice and steady, making it appear as though you’re holding rock-steady power in your workout intervals. It feels clean and accurate.

Workout Graphs Without Erg Mode Power Smoothing

On the other hand, enabling the feature means your trainer isn’t reporting instantaneous power accurately to Zwift. And that can be a bit annoying, especially if you’re a data nerd (and many of us are).

Workout Graphs With Erg Mode Power Smoothing

We aren’t sure how exactly Wahoo’s power smoothing feature works. Is it as simple as reporting, say, 5s average power? Perhaps.

But my main goal today was to shed some light on Wahoo’s Erg Mode Power Smoothing option, so people can disable it if desired. And I wanted to put you at ease if you’ve noticed your power looks more erratic now that you’re on a trainer without this feature. Don’t worry – as long as your trainers are accurate, you’re doing the same work as before. It just may not look as “smooth”.

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